Mechanochemically responsive polymer enables shockwave visualization

Abstract

Understanding the physical and chemical response of materials to impulsive deformation is crucial for applications ranging from soft robotic locomotion to space exploration to seismology. However, investigating material properties at extreme strain rates remains challenging due to temporal and spatial resolution limitations. Combining high-strain-rate testing with mechanochemistry encodes the molecular-level deformation within the material itself, thus enabling the direct quantification of the material response. Here, we demonstrate a mechanophore-functionalized block copolymer that self-reports energy dissipation mechanisms, such as bond rupture and acoustic wave dissipation, in response to high-strain-rate impacts. A microprojectile accelerated towards the polymer permanently deforms the material at a shallow depth. At intersonic velocities, the polymer reports significant subsurface energy absorption due to shockwave attenuation, a mechanism traditionally considered negligible compared to plasticity and not well explored in polymers. The acoustic wave velocity of the material is directly recovered from the mechanochemically-activated subsurface volume recorded in the material, which is validated by simulations, theory, and acoustic measurements. This integration of mechanochemistry with microballistic testing enables characterization of high-strain-rate mechanical properties and elucidates important insights applicable to nanomaterials, particle-reinforced composites, and biocompatible polymers.

Fig. 1. The chemistry and structure of the maleimide-anthracene-functionalized block copolymer (MA-BCP) material.

a Chemical scheme depicting mechanophore activation upon the MA bond rupture due to mechanical deformation. b Schematic of the diblock copolymer with MA mechanophore incorporated between polyisobutylene (PIB) and polystyrene (PS) blocks. c AFM phase image of the sample surface showing spherical features attributed to the PS phase distributed within the PIB matrix. Scale bar for the magnified inset image is 100 nm.

Fig. 2. High-strain-rate microballistic impact test results of the MA-BCP films.

a Time-lapse ultrafast camera images of a representative impact event (interframe time is 230 ns). Images capture the trajectory of a 20 μm silica microprojectile as it impacts and rebounds off the MA-BCP film surface. b Plot of impact (v_i) and rebound (v_rb) microprojectile velocities measured from the ultrafast camera images. c Plot of the kinetic energy loss parameter (ΔKE/KE_i) as a function of impact velocity. Each open circle corresponds to one impact test. The three filled symbols highlight specific impact tests, v_i = 407 ms⁻¹ (purple circle), 414 ms⁻¹ (orange square), and 515 ms⁻¹ (red pentagon), where the results are discussed in greater detail in Fig. 3 and Fig. 4.

Fig. 3. Ex-situ surface characterization of impacted sites on MA-BCP films.

a Schematic of the local deformation induced by the microprojectile on the MA-BCP film with a contact radius (a) and depth (δ). b AFM line scans of three different impacted sites highlighted in Fig. 2b, c. c–e AFM phase images corresponding to impact velocities of 407 ms⁻¹, 414 ms⁻¹, and 515 ms⁻¹, respectively. The dashed line indicates the perimeter of the contact area and the scale bar represents 5 μm. The inset shows a magnified image of the BCP nanostructure taken near the center of the impacted site; the scale bar represents 500 nm. f–h FM images taken on the film surface (z = 0 plane) and corresponding to the contact areas c–e, respectively. The scale bar represents 5 μm.

Fig. 4. Visualization and simulation of shock deformation within the bulk of the MA-BCP films.

a 3D projection of the mechanophore-activated volume below the impacted site corresponding to the impact velocity case, v_i = 414 ms⁻¹. b A 2D slice of the 3D projection, from a shows that the activated volume resembles a Mach cone. The dashed red line indicates the deformed film surface as measured by AFM, and the scale bar represents 5 μm. c Velocity contour map of the BCP material extracted from an FEA simulation of the microballistic impact experiment, with v_i = 407 ms⁻¹. d The Mach cone angle (α) extracted from FM measurements and FEA simulations versus impact velocity (v_i). Data from the FEA simulations are presented as mean values  ± SD. The gray curves are fits to the data that extrapolate the shear wave velocity of the MA-BCP material according to the Mach-cone-angle relationship Eq. (1). e Kinetic energy loss parameter (ΔKE/KE_i) replotted as a function of impact velocity normalized by the shear wave velocity extracted from FM measurements (v_i/C_S). Impact cases in the subsonic regime (v_i < C_S) are fit by the impact model in Eq. (2). Mach cones are only observed in FM measurements for impacts in the intersonic regime (C_S < v_i < C_L) with representative cases highlighted by the triangle symbols.